Setting the default app for a file can be accomplished in a number of ways, such as getting info on the file in the Finder and making your selection. If you want to save yourself a little time, however, just right click on the file and choose "Open With" while holding the option key. This will change the command to "Always Open With" so whatever you choose will be the default app for that particular file. When you make your selection, it'll also open the file in that app so you don't waste a few extra clicks you don't need to make.